1. Field of the Invention
The present invention relates to a tandem-type vacuum booster used for operation of a brake master cylinder or the like of an automobile, and particularly, to an improvement of a tandem-type vacuum booster comprising a partition plate fixedly mounted in a booster shell for partitioning the interior of the booster shell into a front shell chamber and a rear shell chamber; a front booster piston dividing the front shell chamber into a frontside fore vacuum chamber connected to a vacuum pressure source and a rear-side fore operating chamber; a rear booster piston dividing the rear shell chamber into a front-side hind vacuum chamber and a rear-side hind operating chamber, the both booster pistons being interconnected through a piston boss slidably carried on the partition plate and connected to an output rod; a valve tube integrally connected to the piston boss to project from a rear end of the piston boss and slidably supported on a rear wall of the booster shell; a first passage permitting the interior of the valve tube to communicate with both the vacuum chambers; a second passage permitting the interior of the valve tube to communicate with both the operating chambers, the first passage being comprised of an axial passage permitting the communication between the fore vacuum chamber and the valve tube and a radial passage permitting the communication between the axial passage and the hind vacuum chamber; an input rod movable back and forth within the valve tube; and a control valve for changing over the communications of the second passage with the first passage and the atmosphere depending upon the backward and forward movements of the input rod, the input rod and the control valve being disposed within the valve tube.
2. Description of the Prior Art
Such vacuum booster has conventionally been known, for example, as disclosed in Japanese Utility Model Application Laid-open No. 121160/87. In such known vacuum booster, however, when the fore and hind vacuum chambers are put in communication with the fore and hind operating chambers through the first and second passages by the retreating operation of the input rod in order to bring the vacuum booster from an operative state back to an inoperative state, the air hitherto occupaying the operating chambers is rapidly drawn from the second passage via the axial passage of the first passage into the fore vacuum chamber. During this time, the air flows at a high rate through the axial passage from the side of the valve tube toward the fore vacuum chamber. However, it has been investigated by the present inventors that in the prior art booster, a part of the air flowing through the axial port 030A diverts toward the axial port 030R, as shown in FIG. 7, to creat a vortex c in the vicinity of the opening of the passage 030R, which is attributable to an operational noise (an air-cutting-through noise).